underling the correct word in each sentence1)where039;s an/the electric heater?I can039;t

Underling the correct word in each sentence
1)where's an/the electric heater?I can't find it
2)what happened at an/the end of a /the film?
3)David has an/the appointment at a/the opticians
4)an/the old person sometimes feels lonely
5)peter built a/the largest model plane in a/the world
6)Luckily a/the fire brigade soon came and put out a/the fire
7)Harry's mother bought him a/the guitar for his birthday present
8)I'm thinking of buying a/the new pair of trouser
9)in the end there was a/the war between the two countries
10)I didn't know an/the answer to a/the question ,so I left it out